Shock absorber

The shock absorber is a safety- relevant part that can relieve the vibrations of the sprung mass quickly for trolleys. Correctly be called " vibration dampers ", because this device converts the vibration energy into heat. Without this energy conversion, the damped oscillation would decay too slowly.

The shock absorber is not intended to be supported by road irregularities introduced into the vehicle collisions. For this, the suspension is responsible.

• 4.1 friction damper
• 4.2 monotube
• 4.3 twin-tube dampers

Design and function

Hydraulic shock absorbers

Hydraulic shock absorbers consist essentially of a run on a piston rod in an oil-filled cylinder piston. Upon axial movement of the piston rod (and the piston) against the cylinder has to flow through narrow channels and valves in the piston oil. By the resistance, which is the reason that the resulting oil, the pressure differences generated, which generate damping forces over the active areas. The resulting damping work is converted into heating of the oil. The viscosity, and thus the damping effect of the oil is temperature dependent. In order to limit the temperature rise of the damper to a tolerable level for the components involved, the damper must be sufficient heat can be left at the ambient air.

The volume of einsinkenden piston rod must be balanced within the damper. A pure oil damper so it can not give, because oil is like all liquids virtually incompressible. Compensation may, for example, be realized by a high pressure (~ 30 bar) with gas cushion of nitrogen or air, which is arranged separated by a movable piston from the oil volume ( monotube ). By displacement of the dividing piston, the gas cushion takes the volume compensation during retraction of the piston rod. The gas acts as an additional spring, so that the effect of the suspension is supported.

Train and rebound

A directly articulated hydraulic shock absorber is claimed during rebounding to train and during compression to pressure. Therefore, the suspension damping than rebound, during compression is called the compression stage.

In order to improve the " springing " when approaching ramp Einzelhindernissse, the rebound is usually harder than running the printing stage. Another reason for this interpretation is a harmonious construction of the roll angle during rapid evasive maneuvers.

Mechanical Shock

Mechanical shock principally consist of spring-loaded friction surfaces. Laminated leaf springs consist of several spring leaves and represent combined Feder-/Dämpfer-Einheiten, which are preferably used because of their rugged, durable and cost-effective design in trucks. By the deflection of the spring of the shock is absorbed and stored in the spring. The friction between the leaf springs dampen the vibration and generates heat. However, this attenuation is not dependent on the speed at which movement takes place, the spring, and therefore in comparison to the speed-dependent damping of the hydraulic shock absorbers less effective. In general, therefore, additional hydraulic shock absorbers are fitted with leaf springs.

Other forms

A special used in the Formula 1 design is the applied external rotary shock absorber. A new development are the air spring dampers which are installed in both the commercial vehicle sector as well as in passenger cars. You can also take on the level of regulation in addition to the suspension and damping. Also, motorcycles and bicycles are equipped with air spring dampers in which the medium of air both spring and damper tasks takes over.

Importance of the shock absorbers in motor vehicles

After brakes, tires and steering of the shock absorber is the most important component to control a vehicle safely. Nevertheless, it is (for an MOT for example ) performed in Germany as part of the main study, only a visual inspection of the shock absorbers. A function test can be a shock tester (german = shock shock absorber, or in short, shock ) are carried out, in which the individual wheels of the vehicle to be vibrated and then the decay of the vibration is recorded.

By Bumper the tires especially when driving through curves, but even during hard braking on the road are kept. Without their vibration damping, the wheels would rebound automatically after the compression again, thereby accelerate the vehicle up, and thus reduce the normal force of the wheels to the road, which would cause the friction force that can bring the tires to the road surface decreases, . The vehicle then slides ( transition from static friction in: it slip occurs, this can be perceived as a squeak ). Formulated Figuratively ' " hops " the vehicle like a rubber ball on the road. The tires can, however, transfer only drive, braking and lateral forces ( cornering) when they are pressed with a certain force to the road. A vehicle with springs without shock absorber is therefore not certain controllable.

Sometimes it is stated that for vehicles without ABS, the braking distance of 50 km / h to rest is longer by four meters if the shock absorbers are worn out ( this depends on the condition of the road ). With non-functioning shock absorbers doubled the likelihood of a rollover in a Notausweichmanöver (iso - lane change test, elk test ). The life of shock absorbers in cars is dependent on type of use and may 60000-80000 km there.

When designing a vehicle springs and dampers are matched. In practice, retrofit kits from the accessories trade, are where shorter springs ( lowering) combined with the existing damper, lead to poorer driving behavior: If the spring rates are higher, but not change the damping rates to vibrations in the chassis, the vehicle can last longer, " jumps " after driving over bumps. For series chassis with defective dampers, the frequency is somewhat lower, it bobs up and down slowly. In both cases, the maximum achievable speed curve is lower than in intact or matched to the springs shock absorbers at a given radius, as the tires quickly lose contact with the road on bumps because of the vibrations.

Detection of defective shock absorbers in cars

Decreasing damping is often unconsciously compensated by a change of driving habits of the driver. There are some signs of weakening shocks, the effects which occur do not occur abruptly, but go hand in hand with increasing wear of the damper:

• Multiple ringing when shifting the vehicle in vicinity of the wheel by hand in oscillations (simple function test, the behavior is especially evident in dampers that have become completely inoperable )
• After uneven swings the vehicle after
• Thumping noises on rough roads at low speed ( 30 - zone)
• Uneven wear of tires and increased tire wear
• Fluttering steering or braking track often interrupted by a screeching halt because of jumping wheels
• Spongy cornering performance, lane drifts in wavelength the vehicle in response to the excitation of vertical vibrations to the outside
• Increased sensitivity to side winds

Completely defective dampers can also be seen by substantial amounts oil exiting the piston rods of the damper. Conversely, it can be deduced but from a completely dense shock does not function properly.

Types of shock absorbers in cars

When the shock absorber designs in cars, a fundamental distinction between axle damper, that is a stand-alone vibration and shock, mainly in the design of a McPherson strut. Conventional shock absorbers in cars are now manufactured mainly in hydraulic version in one- and two-pipe design.

Friction damper

Before the development of the hydraulic shock absorber, the vehicles are equipped with friction dampers. These allow decay vibrations faster but seem undesirable federungsverhärtend: At rest, there is a friction between the friction surfaces, which must be overcome by a greater force ( breakout ). This occurs not only in front of the excitation, but may on the apexes of the motion during the vibration process.

Friction damper consisted of two nested cylinders and several steel plates and friction pads. The outer cylinder was fixed to the structure and the steel discs were secured in the cylinder from turning. The friction pads were fixed to the inner cylinder, and this via a linkage to the vehicle axle. The arrangement of alternating steel plates and friction pads, the operation is equivalent to a multi-plate clutch. By changing the bias of a spring inside the damping ratio could be adjusted.

Monotube

The single-tube is subdivided into the working chamber ( oil chamber ) and counter- pressure chamber ( gas chamber ). In the oil space, the actual damping work is done, ie the seated on the piston damping valves set the oil flowing through the piston acts as a resistor. Wherein a pressure differential is created that opposes the container moving relative to the piston rod, a damping force. The gas chamber is used to volume and temperature compensation. During compression, the gas chamber compensates the volume displaced by the piston rod of the amount of oil by compression. For heat generation by the damping piston on the damper oil will expand. This extension is also compensated for by the volume of gas. Typically, a single-tube has a base pressure of 20 - 30 bar. This bias voltage is needed so that the compression is not in the upper working chamber breaks off the oil column ( chamber above the piston), and therefore no vacuum is created (risk of cavitation). This would adversely affect the Dämpfkraftcharakteristik the damper.

Twin-tube dampers

The twin-tube dampers has in contrast to the single-tube in addition to the cylinder tube in which the mounted on the piston rod and stocked with other parts of the valve piston moves axially, another coaxially arranged container tube. The piston divides the inner oil chamber into an upper and lower work space. In the compression stage, the piston rod retracts and flows some of the oil from the lower working chamber through the piston valve into the upper working space. That of the dipping piston rod corresponding to the oil volume is pressed by an onboard the lower end of the cylinder tube bottom valve in the so-called compensation chamber between the cylinder and the container tube. In this case, a relevant for the damping pressure difference is also generated by the bottom valve. When the piston rod (rebound ) the piston valve assumes the damping, while that of the extending piston rod oil volume in back flow largely unimpeded through the bottom valve.